The invention relates to a method for molding a plastic article of a desired density, and more particularly, to a method for molding a plastic article with a foamed interior and a smooth dense outer skin of a desired thickness.
Plastic injection molding machines generally include a two section mold unit wherein one of the mold sections is stationary and includes an end opening for the injection of mold material into a cavity. The other mold section is generally movable between an open position away from the stationary mold section and a closed position wherein the two mold sections are in sealed engagement to form a mold cavity.
Once the mold cavity has been formed, a screw or similar injection device is used to inject a plastic material into the mold cavity where the material is cured under pressure. It is well known in the prior art to combine a blowing agent with the plastic material to decrease the density of the finished plastic part.
Injection molding provides efficient means for producing plastic articles both quickly and economically. It is often desired to create a plastic article of a decreased density, as the decreased density reduces the amount of material needed to create an article of a given dimension, and also reduces the weight of an article of a desired dimension. One way of reducing the density of the plastic article is to combine a blowing agent with the plastic material. Most blowing agents are heat-activated, and produce bubbles, or foam, within the plastic when activated. If the blowing agent is distributed evenly throughout the plastic material, the bubbles produced by the blowing agent will generally be evenly distributed throughout the plastic material after the blowing agent is activated. As the plastic material cools and hardens, these bubbles form a cellular structure throughout the plastic article.
Although it is desirable to have this cellular structure within a plastic article, it is desirable to have a more dense outer skin that is free from the cellular structure created by the blowing agent. This outer shell gives the plastic article a more aesthetic appearance and a tougher covering.
It has been shown in U.S. Pat. No. 5,437,823 how to make a plastic article having a dense unfoamed outer skin and a foamed interior. This is achieved by injecting the plastic material and blowing agent into the mold cavity under a high enough pressure to prevent the blowing agent from forming bubbles. The plastic is maintained at a high pressure while a skin forms along the walls of the mold cavity. The pressure is then decreased, and the blowing agent is allowed to foam the interior of the plastic article.
While it has been shown how to make an article with a dense skin and a foamed interior, the process has not been adjustable to accurately control the thickness of the outer skin in a molded plastic article. It is desirable to control this thickness as it allows articles of a precise weight, dimension, density, and strength to be produced. Being able to accurately control the skin thickness is also valuable in creating an angled plastic article made from a single flat sheet of a plastic article, as it allows precise control of the bending radius of the outer shell. Molding fabric into the surface of a plastic article for support or decoration is also aided by being able to accurately control the skin thickness of a controlled density plastic article having a dense outer skin and a foamed interior.